| Year | Value | | 2021 | 1.3 | | 2020 | 1.32 | | 2019 | 1.39 | | 2018 | 1.4 | | 2017 | 1.4 | | 2016 | 1.39 | | 2015 | 1.53 | | 2014 | 1.54 | | 2013 | 1.51 | | 2012 | 1.49 | | 2011 | 1.61 | | 2010 | 1.59 | | 2009 | 1.52 | | 2008 | 1.63 | | 2007 | 1.44 | | 2006 | 1.42 | | 2005 | 1.55 | | 2004 | 1.57 | | 2003 | 1.39 | | 2002 | 1.5 | | 2001 | 1.57 | | 2000 | 1.55 | | 1996 | 1.5499999523 | | 1993 | 1.6000000238 | | 1990 | 1.6440999508 | | 1980 | 1.9071999788284 | | 1975 | 2.5462999343872 |
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